A bipolar junction transistor (BJT) is common type of transistor that is generally used in amplifying or switching applications. BJTs are typically three terminal transistors, which have a base, collector, and emitter. An exemplary unit cell structure for a vertically stacked NPN-type BJT 10 is illustrated in FIG. 1. As depicted, the BJT 10 includes a substrate 12, which is heavily doped with an N-type dopant (N+) and formed from a portion of a semiconductor wafer 14. A collector 16, which is moderately doped with an N-type dopant (N), is formed over the substrate 12 from one or more collector layers 18. A base region 20, which is moderately doped with a P-type dopant (P), is formed over the collector 16 from one or more base layers 22. An emitter 24, which is heavily doped with an N-type dopant (N), is formed over a central portion of the base region 20 from one or more emitter layers 26.
An emitter cap 28, which is more heavily doped with an N-type dopant (N+) than emitter 24, is formed over the emitter 24 from one or more emitter cap layers 30. An emitter ohmic contact 32 is formed on the emitter cap 28. The emitter cap 28 and the emitter ohmic contact 32 effectively form an electrical contact for the emitter 24, wherein the emitter ohmic contact 32 facilitates external electrical connections to the emitter cap 28, and the emitter cap 28 provides a relatively low resistance connection to the emitter 24.
The contacts for the base region 20 may be formed by selectively heavily doping outer portions of the base region 20 with a P-type dopant (P+) to form base cap regions 34 within the base region 20. Base ohmic contacts 36 may be formed on the base cap regions 34 to facilitate external electrical connections with the base cap regions 34, wherein the base cap regions 34 provide relatively low resistance connections between the base region 20 and the respective base ohmic contacts 36. Alternatively, each base cap region 34 may be formed from a separate layer that resides on an upper surface of the base region 20, as opposed to being provided in the base region 20 as depicted.
A collector ohmic contact 38 may be formed on the bottom side of the heavily doped (N+) substrate 12 to provide a contact for the collector 16. In essence, the collector ohmic contact 38 facilitates external electrical connection to the substrate 12, which provides a relatively low resistance connection between the collector 16 and the collector ohmic contact 38. Alternatively, the collector ohmic contact 38 may be formed on a collector cap (not shown) that is formed on the upper surface of the collector 16 or within the collector 16.
In operation, the BJT 10 allows a collector current is to flow from the collector ohmic contact 38 to the emitter ohmic contact 32 through the base region 20 when forward biased. Being forward biased means that a positive voltage of sufficient magnitude is applied across the base ohmic contact 36 and the emitter ohmic contact 32. In addition to the collector current ic current flowing from the collector ohmic contact 38 to the emitter ohmic contact 32, a relatively small base current ib flows from the base ohmic contacts 36 to the emitter ohmic contact 32, as illustrated in FIG. 2A. The base current ib flows laterally from each of the respective base cap regions 34 inward toward the portion of the base region 20 that resides beneath the emitter 24 and then flows vertically upwards through the emitter 24 and the emitter cap 28 to the emitter ohmic contact 32. The base region 20 is somewhat resistive, and as such, the lateral flow of the base current ib through the base region 20 generates a lateral potential difference, or voltage drop, so-called self de-biasing, in the base region 20. In other words, the potential throughout the base region 20 varies, and in particular, gradually increases from a central region RC, which resides under the middle portion of the emitter 24, to the respective outer regions RO.
As illustrated in FIG. 2B, the lateral potential difference in the base region 20 causes a significantly uneven distribution of the collector current ic through the emitter 24 and the portion of the base region 20 that resides below the emitter 24 when the BJT is forward biased. As a result, the relatively lower potentials at or near the central region RC cause a relatively lower concentration of collector current ic to flow through the central region RC of the base region 20 and the central portion of the emitter 24. Conversely, the relatively higher potentials in the base region 20 beneath the outer portions of the emitter 24, cause relatively higher concentrations of collector current ic to flow through the outer portions of the base region 20 and emitter 24. The density of collector current ic continues to increase as the outer edges of the emitter 24 are approached. The circled portions that are labeled “A” highlight the outer areas of the base region 20 and the emitter 24 where collector current ic density is highest. The phenomenon of having significantly higher density of collector current ic near the outer areas of the base region 20 that reside beneath the emitter 24 and the outer areas of the emitter 24 is referred to as “emitter current crowding.”
Current crowding is problematic in BJTs because the excessive collector current ic density in those areas that are prone to current crowding generate excessive amounts of heat. The excessive heat generation in those areas that are prone to current crowding leads to poor device performance, and in many instances, permanent damage. As such, there is a need to reduce current crowding in BJTs. There is a further need to reduce current crowding in BJTs without significantly impacting overall performance of the device.